An electrode layer of a built-up condenser (or laminated condenser) or other electronic parts is generally prepared by coating an electroconductive paste which comprises a precious metal powder (such as silver powder, platinum powder, gold powder, or palladium powder) and an organic binder on a ceramic substrate and firing the coated layer. Thus prepared electrode layer is a continuous layer essentially consisting of the precious metal. The continuous layer of precious metal shows low electric resistance and high electroconductivity. Therefore, such precious metal electrode layer has been conventionally employed.
The built-up condenser comprises at least several condenser units (in some cases, condenser units of more than one hundred) in which each condenser unit has an electrode layer formed a ceramic substrate (dielectric substrate). Therefore, each of the substrate and electrode layer for the use of the preparation of a built-up condenser should be as thin as possible. For instance, in a recently prepared built-up condenser comprising condenser units (each being composed of a substrate and an electrode layer) of several tens, one electrode layer generally has a thickness of approximately 1 .mu.m or less.
Various processes for preparing a built-up condenser comprising a large number of condenser units have been known. Most generally employed process comprises laminating several tens of unfired ceramic substrates (i.e., green sheet or raw sheet) coated on their surfaces with an electroconductive paste (which is a mixture of a precious metal powder and a spreading agent containing an organic binder) one on another, and firing the laminated body so that firing of the unfired substrates and burning of the organic binder in the coated layers can be simultaneously done to give the desired electrode layers.
As material of the ceramic substrate of built-up condensers, barium titanate or titanium dioxide is generally employed, because these materials have good dielectric characteristic and physical properties. As material of the electrode, palladium is generally employed because palladium sinters at a temperature almost equivalent to the sintering temperature (approximately 1,200.degree. C.) of barium titanate or titanium dioxide.
Palladium, however, has a drawback in that a palladium powder shows noticeable volume expansion within a short time of period due to rapid oxidation on its surface when it is heated to about 400.degree.-900.degree. C. in air. When such expansion occurs, a composite of several tens of units each of which comprises an electroconductive paste layer comprising a palladium powder and an unfired ceramic substrate is deformed in its thickness direction (i.e., depth direction) in the firing process due to rapid expansion of the electroconductive layer. Thus oxidized palladium powder decomposes to release oxygen to form a palladium electrode layer after firing to 1,000.degree.-1,200.degree. C. The expansion of the sintered electroconductive paste layer in the thickness direction by the surface oxidation of palladium powder sometimes occurs nonuniformly over the paste layer. Therefore, if the oxidation and expansion of the palladium powder occurs rapidly, structural defects such as delamination and crack are produced in the resulting electrode layer. Further, the thickness sometimes varies locally in the electrode layer. If such structural defects as delamination and crack are produced in the process for preparing a built-up condenser or if the formed electrode of a built-up condenser has nonuniform thickness, the condenser sometimes shows wrong electric characteristics and is failed to requirements. Thus production yield lowers.
Heretofore, the oxidation and expansion of the palladium powder in the electroconductive paste and the structural defects and deformation of the electrode layer caused by the oxidation and expansion are suppressed by controlling the firing conditions (for instance, prolonging the firing period). However, the suppression of the oxidation and expansion by the conventional measures are not sufficient. Moreover, the prolongation of the firing period is disadvantageous in the industrial production.